Flooring installations comprising a hard, brittle flooring material such as tile or terrazzo on a rigid coherent base (typically concrete) have been widely used for years in public buildings, such as shopping malls, banks and office buildings. Such flooring installations are used above, at and below grade. In the latter two cases (at or below grade), a concrete base or slab is typically supported on a sub-base, i.e. the ground. A major problem is encountered in such installations. Concrete shrinks and generally cracks as it cures. In at grade and below grade installations, such shrinkage and cracking result in dimensional changes in the horizontal (or lateral) direction. This sets up stresses between the concrete base or slab and the sub-base (the ground) and between the concrete base and the flooring. In the case of a direct bonded flooring installation (one in which the flooring material is firmly bonded to the base), lateral shifts in the base are broadcast upwardly to the flooring material and may cause the flooring to crack. In a structural deck (i.e., an above ground installation), cracks tend to be small (and more frequent) due to the use of reinforcing bars in the concrete base. However, the concrete base deflects slightly, and this deflection or sagging places the flooring material under stress and may cause cracking.
Prior to and shortly after World War II, most ceramic tile and terrazzo installations utilized "mud setting" beds. These beds were composed of a lean mixture of sand and cement, placed fairly dry and generally not bonded to flooring base surface. Typically the mud setting bed was separated from the base with 15 pound roofing felt or the like. Tiles were fairly thick, e.g. 3/4 inch to 11/4 inches thick and, the mud beds were generally in the range of about 11/4 to 11/2 inches thick. The same basic systems were used for terrazzo flooring.
FIG. 1 shows a representative mud bed installation. In FIG. 1, mud bed installation 30 comprises a base 32 of hard coherent material, usually concrete (or alternatively, wood); a top flooring 34 of tile, terrazzo or other hard, brittle flooring material which is in the form of individual "squares" (or pieces of other simple geometric shape). The flooring material pieces are held together by means of grout 36 in the spaces between adjacent tiles, and a mortar layer 38 which is applied to the underside of the tiles. The tiles 34 are typically about 3/4 inch to about 11/4 inches thick. Between the mortar 38 and the concrete base 32 are a "mud" bed 40 (a dry-pack sand/cement mortar setting bed) which is typically about 11/4 inch to 11/2 inch thick, and (between the mud bed and the concrete base) a sheet or membrane 42 of a compressible material such as 15 pound roofing felt or the like.
Since the flooring system was not bonded to the base, the base was free to move laterally with respect to the rest of the system. Although this created some problems, it also offered the significant advantage that both the tile and the base (when a concrete base was used, which was typical) was protected from cracking. Shear forces caused by horizontal movement of the base were not transferred to the top flooring surface. In addition, the very thickness of the system permitted a transfer of live impact loads on the top flooring system to dissipate to minimal levels prior to reaching the base level. (An "impact load" as the term is used herein denotes a high-stress on the top flooring surface. It is typically caused by a sharp object striking the flooring surface).
Such live load impact force on the top flooring 34 is shown by the downward arrow "F" in FIG. 1. Two sets of broken lines extend downwardly and outwardly from the point of impact of force "F" in FIG. 1. These represent the area over which the force "F" is dissipated as it is broadcast downwardly through the flooring structure. The dissipation angle would ordinarily range from 45.degree. (represented by the inner pair of broken lines) to 60.degree. (represented by the outer pair of broken lines). As is evident, the force "F" is greatly attenuated by the time it reaches space 32, due to the thickness of the flooring 34 and the bed 40.
Beginning in the early 1950's, the thick tile floor systems described above gave way to thin set systems, utilizing much thinner tiles, rarely over 1/2 inch thick. Flooring systems of this type were less costly, lighter and are more easily coordinated with installations of carpet or vinyl flooring.
Modern thin-set flooring installations are of two general types. In one type, shown in FIG. 2, tile or other hard flooring material (such as terrazzo) is direct bonded to the base. In FIG. 2, a direct bonded installation of this type comprises a concrete base 32, a top flooring layer 34 of tile in the form of individual pieces or "squares", which are held together by means of grout 36 between adjacent pieces and mortar 38 applied to the underside of the tiles. (The flooring layer 34 also has expansion joints not shown). The mortar 38 is in direct contact with the concrete base 32 (hence the term "direct bonded"). The top flooring layer 34 may have expansion joints. (Also, the concrete base 32 may have control joints which prevent the spread of cracks). Because of the thinness of the installation, stresses at the interface between mortar 38 and concrete base 32 due to a live impact load on the tile or terrazzo 34 are much greater than is the case in a mud bed installation as shown in FIG. 1. Within the general field of the flooring structure 30 (i.e., away from the edges of the flooring structure and away from any joints in either the base 32 or the flooring 34), the concrete base 32 supports both the dead weight load of the flooring and mortar thereabove as well as any vertical live impact loads (except those which exceed the impact strength of the flooring material). However, there are horizontal stresses, especially at interfaces (e.g. between the base 32 and the mortar thereabove) in the flooring structure, and failure of the flooring layer may occur due to these horizontal stresses. Also, concrete shrinkage cracks are broadcast to the top flooring material. This type of system has very little "give" permitting relative lateral movement between the top flooring material 34 and the base 32. A direct bonded flooring structure employing terrazzo flooring will be generally be similar to that shown in FIG. 2, except that the terrazzo pieces are typically set in a mortar matrix and grout 36 and a separate bed mortar layer 38 are not required.
Shrinkage or cracking in the concrete base will therefore lead to failures in the flooring layer 34.
Systems of the type shown in FIG. 2 are the most widely used type of tile or terrazzo flooring systems in buildings built from the 1950's to the present.
A less widely used system is shown in FIG. 3. The flooring system 60 therein differs from that shown in FIG. 2 in that it includes a thin membrane 62 interposed between the mortar layer 38 and the base 32. Such system insulates the flooring tiles from cracks in the concrete base. On the other hand, flooring tiles are even more vulnerable to cracking due to impact load than is the case in the direct bonded system such as shown in FIG. 2. This is because the relatively resilient membrane 62 permits the bottom surface of the mortar layer 38 to be placed in tension when the flooring is struck by an object which causes an impact load. As is well known, mortar, concrete and ceramic materials have high compressive strength but low tensile strength. Hence, an installation such as that shown in FIG. 3 is quite vulnerable to impact loads.
Applicant's co-pending application, Ser. No. 07/433,656, filed Nov. 8, 1989, now U.S. Pat. No. 5,052,161, issued Oct. 1, 1991 provides a structure which greatly minimizes tile or terrazzo cracking due to either impact load or dimensional changes in a concrete or wood base in a thin-set system. Basically, a thin plastic crack isolation sheet having upstanding dimples is interposed between the concrete or wood base and the tile bed mortar on the underside of the tile or terrazzo layer, so that the base of the sheet is on top of the wood or concrete base and the sheet's dimples rise upwardly from the base. The space between the sheet base and the tile bed mortar, which is not occupied by dimples, is filled with mortar which may be called compression bed mortar. This crack isolation structure comprising the above-described crack isolation sheet and compression bed mortar, with air space beneath the dimples, provides support from the concrete (or wood) base for the tile (or terrazzo) top flooring layer, so that the flooring layer can withstand substantial impact loads, and also protects the flooring layer (tile or terrazzo) from stresses due to lateral shifting (occasioned by shrinkage or cracking) of the base. This compression bed mortar may become cracked at the stress site, but this protects the crack from spreading further and the compression bed mortar, even when cracked, retains its compressive strength and ability to function. Applicant now believes that the ability of this system to prevent cracks from spreading is due primarily to the compression bed mortar.